There's usually games open in Survivors. It's horribly unbalanced but it's still fun. But it's even better if you find people who play fair. I'd say it's worth it.
Predator's fun as well but survivors is better imo. Ustanak is hard to play at first but it doesn't take long to get the hang of it. Sometimes it's empty, sometimes there's a game open. It's definitely worth it.
Onslaught is pretty dead but occasionally you'll find a game or two. If not you can just create your own game and it doesn't take long to fill up since you only need two people. If you're a fan of mercenaries then it's definitely worth it.
Siege is completely dead. Last time I played this was months ago. It's a shame because imo this is the most fun out of all the modes. If you're looking to play with random people most of the time, I'd recommend you to skip it since it's empty 90% of the time, but if you can find people to play with then it's a must-have.
GT is Necrobulll btw. Always looking for people to play with.